In disk drives that employ accessing magnetic heads, it is generally necessary to provide means to maintain the head substantially over the center of a selected data track to obtain an optimum readout signal. For this purpose, track following servo systems have been used to compensate for position error and to drive the head towards the center of the track whenever displacement occurs. In prior art servo systems, separate servo signals are registered either on a separate disk surface or in servo sectors on the disk surface utilized for recorded data. Such arrangements require separate servo heads, or suffer from reduced storage capacity necessitated by dedicating disk surface area for servo signals.
In U.S. Pat. No. 3,246,307, a servo positioning system is disclosed that employs a magnetic head with two separate spaced magnetic elements for sensing data tracks as the source of servo information. However, when operating with disk drives that employ relatively high track densities and narrow track widths, the spacing, alignment and attitude of the separate magnetic elements are critical for deriving accurate readouts of recorded signals. Such requirements impose tight tolerances and stringent manufacturing processes which make production and maintenance costly, uneconomical and not feasible.